High-altitude erratics in the Bernese Alps (Switzerland)

Abstract

Little is known about the Early Pleistocene landscape and glacial history of the Swiss Alps, largely because of the scarcity of sediments dating from that period. Here we investigate high-altitude, presumably Early Pleistocene relics of unconsolidated, near-surface sediments that occur at the Stockeseen site (close to the Stockhorn) and at Wagenmoos (close to Sibe Hängste) in the Bernese Alps. We complemented our study by analysing cave gravels from 13 sites. Whereas the underlying bedrocks are part of the purely sedimentary Penninic Prealps and the Helvetic zone, the investigated sediments and cave gravels contain characteristic erratic crystalline clasts (HP-LT-metagabbro, medium-grade metamorphic quartzite, jadeitite, glaucophane-schist, low-grade metamorphic gabbro and peridotite). The erratics originate from Penninic and Austroalpine nappes which are exposed only south of the Bernese High Alps, today’s water divide. In combination with partly distinct glacial features of the indicator erratics, this suggests that transfluences existed from the Valais (Rhône valley) to the Bernese Alps. Our findings suggest two transfluence routes, one over a precursor of the Gemmipass to the palaeo-Kander valley, providing crystalline erratics towards Sibe Hängste, and one over precursors of the Sanetschpass and Saanenmöserpass into the palaeo-Simmen valley and towards the Stockhorn. The Wagenmoos erratics must have been deposited before the re-routing of the palaeo-Aare river (from northward to westward) and its subsequent deepening, which indicates an Early Pleistocene timing of the respective transfluence. This is in agreement with published burial ages of ~1.87 Ma ± 0.21 for cave gravels with crystalline components in the cave system Réseau Siebenhengste–Hohgant (beneath the Wagenmoos site).

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Acknowledgments

We are particularly grateful for experienced help in rock identification from J. Abrecht, H. Bläsi, M. Herwegh, B. Hofmann, T. Labhart, H. Masson, I. Mercolli, J. Meyer, H.-R. Pfeifer and W. Winkler. Many discussions with A. Breitschmid, W. Flück, U. Menkveld-Gfeller and A. Pfiffner concerning the regional geology were very beneficial to the work. Special thanks go to the speleologists P. Häuselmann, A. and F. Hof and P.-Y. Jeannin. Their support and reference collections enabled the investigation of cave gravel. P. Häuselmann, F. Preusser and, in particular, R. Zech are thanked for the reading of an early version and for constructive suggestions which significantly improved this manuscript. The paper has greatly benefited from the reviews of M. Fiebig and A. Dehnert.